The present invention relates to transmission systems and, more particularly, to a method of scheduling links for simultaneous transmission over a single resource based upon dynamic characteristics of the transmission medium.
In wireless communications systems, several techniques are known to isolate links from one another. The conventional advanced mobile phone system (AMPS) uses frequency division multiple access (FDMA) techniques to isolate calls. FDMA isolates calls by assigning each call within a cell to a unique pair of RF channels. No two calls within a cell are permitted to share the same channel assignment. Consequently, the calls do not interfere. A second technique, time division multiple access (TDMA), builds upon the advantages of FDMA by dividing an RF channel into repeating frames, each frame containing a predetermined number of time slots. Individual calls on the same RF channel are assigned to different time slots, thereby ensuring that the calls do not mutually interfere. A third technique, code division multiple access (CDMA), modulates each call with a code that uniquely identifies the call from others that may be transmitted on the same frequency. CDMA receivers decode only the call that is modulated with the code that uniquely identifies the proper call. CDMA may or may not be used with FDMA and TDMA techniques.
A fourth technique, frequency-hopped spread spectrum (FHSS), assigns to each call a periodic sequence of frequencies for use in transmission alternately in succession. Such a sequence of frequencies is called a "hop-sequence." This system is based on the principle that if each user uses a different (preferably random) hop-sequence, dominant interferers are removed and the average interference seen by a call over a hop period is reduced. Call transmissions are demodulated by knowing the hop-sequence assigned to that call.
In these known cellular systems, the resources that isolate individual calls (channels in FDMA, time slots in TDMA, codes in CDMA, hopping sequences in FHSS) may be reused in other cells. Reuse permits two transmitters that will not interfere due to signal attenuation and multi-path interference, to use the same resource. Generally, adjacent cells are not permitted to use the same resource because their proximity encourages interference. Reuse techniques are not unique to cellular applications; for example, public broadcast FM radio reuses channel assignments in various geographically spaced cities throughout the United States. In operating systems, the rate of reuse is a static parameter that is established upon system installation.
However, dynamic reuse techniques currently are subject to investigation in research. Such techniques typically involve a modeling of the transmission medium into a path gain matrix. The matrix may be simplified into a matrix of pairwise interference relationships under an assumption that interference received over a given link is dominated by a single interference source. The resultant binary combinatorial matrix simplifies calculation and scheduling of links for transmission. However, because such an assumption is not necessarily correct, it may lead to less than optimal throughput.
Accordingly, there is a need in the art for a scheduling scheme in a transmission system having dynamic reuse that achieves higher throughput than known schemes. Further, there is a need in the art for a transmission scheduling hierarchy that determines whether a scheduling scheme provides system stability.